1. Field of the Invention
The present invention relates to a surface acoustic wave filter preferably used as a band-pass filter in, for example, a communication system, and more particularly, to a surface acoustic wave filter having a surface acoustic wave resonator that is connected to a longitudinally-coupled resonator-type surface acoustic wave filter.
2. Description of the Related Art
In recent years, in portable telephone systems, with the increase in subscribers and the diversification of services, systems in which the transmission-side frequency band and the reception-side frequency band thereof are close to each other, are increasing in number. Depending on the system, it may be necessary to increase the attenuation value in the immediate vicinity of the pass band thereof in order to prevent mutual interference with other communication systems. Hence, in the surface acoustic wave filter widely used as a band-pass filter at the RF stage of a portable telephone, it is strongly desired to provide an attenuation region in the immediate vicinity of the pass band.
On the other hand, in order to reduce the number of components, there have recently been strong demands that two surface acoustic wave filter elements be combined in one package and the input terminals and/or the output terminals thereof be made common, or that the surface acoustic wave filter with a balanced-to-unbalanced conversion function, so-called balun function, be provided. Recently, therefore, longitudinally-coupled resonator-type surface acoustic wave filters which are easily adaptable to achieve balanced-to-unbalanced conversion, have been widely used as band-pass filters at the RF stage of portable telephones.
For example, Japanese Unexamined Patent Application Publication No. 05-267990 discloses a longitudinally connected double mode SAW filter, as an example of such a longitudinally-coupled resonator-type surface acoustic wave filter.
Also, Japanese Unexamined Patent Application Publication No. 10-126212 discloses a filter having a ladder circuit configuration which includes a longitudinally-coupled resonator-type surface acoustic wave filter.
However, the longitudinally-coupled resonator-type surface acoustic wave filter disclosed in the Japanese Unexamined Patent Application Publication No. 05-267990 has a disadvantage in that the steepness of the attenuation-frequency characteristic in the immediate vicinity of the higher frequency side of the pass band is insufficient. This raises a problem in that the attenuation value on the higher frequency side of the pass band, which attenuation value is required in a PCS system or other similar system, cannot be secured, particularly at the immediate vicinity of the pass band.
In contrast, a surface acoustic wave filter disclosed in the Japanese Unexamined Patent Application Publication No. 10-126212 has been able to increase the steepness of the attenuation-frequency characteristic in the immediate vicinity of the higher frequency side of the pass band. However, the ladder type surface acoustic wave filter in the Japanese Unexamined Patent Application Publication No. 10-126212 has not been able to perform a balanced-to-unbalanced conversion function.
That is, although the need for a surface acoustic wave filter which is superior in the steepness of the attenuation-frequency characteristic in the immediate vicinity of the higher frequency side of a pass band, and which has a balanced-to-unbalanced conversion function has been recognized, such a surface acoustic wave filter has not yet been realized.
In order to overcome the problems with the prior art and to finally satisfy the long felt need for a surface acoustic wave filter which is superior in the steepness of the attenuation-frequency characteristic in the immediate vicinity of the higher frequency side of a pass band, and which has a balanced-to-unbalanced conversion function, preferred embodiments of the present invention to provide a surface acoustic wave filter which exhibits superior steepness of the attenuation-frequency characteristic in the immediate vicinity of the higher frequency side of a pass band, and which is capable of readily performing a balanced-to-unbalanced conversion function.
A surface acoustic wave filter in accordance with a preferred embodiment of the present invention includes a longitudinally-coupled resonator-type surface acoustic wave filter having at least two interdigital transducers disposed on a piezoelectric substrate along the propagation direction of a surface acoustic wave, and at least one surface acoustic wave resonator connected between an input terminal and/or an output terminal and the longitudinally-coupled resonator-type surface acoustic wave filter. In this surface acoustic wave filter, a pass band is formed by utilizing at least one of the resonant modes of the longitudinally-coupled resonator-type surface acoustic wave filter and the inductance of the surface acoustic wave resonator.
In one aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the antiresonant frequency of the surface acoustic wave resonator is preferably positioned at a frequency lower than that in the resonant mode positioned on the highest frequency side, among the resonant modes of the longitudinally-coupled resonator-type surface acoustic wave filter.
In another aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the resonant frequency of the surface acoustic wave resonator is preferably a frequency that is higher than that in the resonant mode positioned on the highest frequency side, among the resonant modes which constitute the pass band of the longitudinally-coupled resonator-type surface acoustic wave filter.
In still another aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the resultant impedance of the surface acoustic wave resonator and the longitudinally-coupled resonator-type surface acoustic wave filter is substantially matched with the impedance in the resonant mode of the longitudinally-coupled resonator-type surface acoustic wave filter.
In a further aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the VSWR (voltage standing wave ratio) at the frequency at the attenuation pole generated by the resultant impedance of the surface acoustic wave resonator and the longitudinally-coupled resonator-type surface acoustic wave filter is preferably at least about 3.5.
In a yet further aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the surface acoustic wave resonator is subjected to weighting by electrode-finger withdrawal.
In another aspect of various preferred embodiments of the present invention, the surface acoustic wave resonator preferably has a smaller electromechanical coupling coefficient than that of the longitudinally-coupled resonator-type surface acoustic wave filter.
In still another aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, the surface acoustic wave resonator preferably includes a piezoelectric substrate which has a smaller electromechanical coupling coefficient than that of the longitudinally-coupled resonator-type surface acoustic wave filter.
In a further aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, at least one inductance element is preferably connected in parallel with the input and/or output terminal to which the surface acoustic wave resonator is connected in series.
In a yet further aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, there are provided at least two surface acoustic wave filter elements. At least one of the input terminal side and the output terminal side of the at least two surface acoustic wave filter elements is made common, and at least one of the two surface acoustic wave filter elements is constituted of a surface acoustic wave filter in accordance with other preferred embodiments of the present invention described above.
In another aspect of the surface acoustic wave filter in accordance with various preferred embodiments of the present invention, a balanced-to-unbalanced conversion function is provided.
A communication device in accordance with another preferred embodiment of the present invention includes a surface acoustic wave filter in accordance with the other preferred embodiments of the present invention described above.
The above and other elements, features, characteristics and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.